I find it curious that you think changing “lenses” matter on that particular topic. There were plenty of people since the founding of this country try who felt that slavery (sure, we’ll go with that example) was wrong. We’re not discussing a topic that everyone thought was benign. Neither was murdering Emmett Till or besting Freedom Riders or exploiting anyone something that was considered proper or right. Neither was anyone confused by the morality of it all.
Moreover excusing past bad behaviors because “that’s the way people thought” seems strange to me, as I assume you believe in an absolute truth. How society viewed certain actions, and excusing those actions (well, that was a different time) is the textbook definition of relativism. From an ethical standpoint at least.